1. Field of the Invention
This invention relates to the free radical addition polymerization of photocurable compositions which contain ethylenically unsaturated compounds. In one aspect, the invention relates to a class of additives useful in such a polymerization and in another aspect the invention relates to the use of these additives to promote essentially complete vinyl polymerization of the composition.
2. Description of the Prior Art
Photocurable compositions are useful in ink and coating formulations. Incomplete cure of photocurable compositions is a problem long recognized in the art. Although this problem is manifest in many photocurable compositions, the problem of incomplete cure with multifunctional acrylate materials is particularly troublesome. Multifunctional acrylate diluents, such as 1,6-hexanediol diacrylate, trimethylol propane triacrylate or pentaerythritol triacrylate, are well-known to produce significant residual unsaturation in photocurable compositions. A similar but lesser known problem can also arise with sterically encumbered monofunctional acrylates such as dicyclopentadiene acrylate (DCPDA). Residual acrylate in cured coatings has several deleterious effects among which are objectionable odors and loss of film properties over time (films may become yellow and brittle). This odor problem is particularly noticeable in the case of DCPDA.
Although numerous approaches are known for essentially completing the cure of such photocurable compositions, many are not effective with sterically encumbered monofunctional acrylates while others have undesirable limitations and/or features. For example, the use of various photoinitiators, comonomers, and drier metals, such as cobalt naphthenate, do not reduce the level of residual DCPDA below 5 or 6 weight percent of the cured compositions. Other means, such as post-bake treatment, have limited utility. Post-baked treatment is generally limited to heat insensitive objects and requires an extra, energy-consuming process step.
U.S. Pat. No. 4,077,858 teaches the use of an aliphatic saturated ether as a promoter for essentially complete polymerization of residual unsaturation in multifunctional acrylate photocurable compositions. However, the method taught therein is somewhat inefficient due to the significant amount of promoter it requires.
U.S. Pat. No. 4,209,371 teaches the use of polyesters of molecular weights not in excess of 1,000 as promoters for essentially complete vinyl polymerization of residual unsaturation in sterically encumbered monofunctional acrylates used in photocurable compositions. However, the method taught does not reduce the level of residual DCPDA to a level which is desirable for some applications.
In view of the aforementioned deficiencies of conventional photocurable compositions, it is highly desirable to provide a photocurable composition that can be more completely cured to a polymeric state having essentially no odor of monomer and having improved physical properties which do not change with time after curing is completed.